MM it's prolly bcoz you workout so often. I hit it 2 days, take a day off, repeat. Even ON, if I try to hit it 3 days in a row, that 3rd day workout suffers. Hence, I make sure I am always recovered so I am 100% full blast ready to kill it when I AM there. Even after a day rest, if I still feel like I just don't "have it", I will take an extra day rest. Keep in mind though I do train very hard AND heavy, mostly HIT, so recovery is key. I never have been the guy who just goes to "get it in", even though having to use light weights or such, just to say I was there. If I don't feel 100% and ready to at least TRY to set a PR on some lift, I'm not going. That's just me - all or nothing.
If you're talking about short-term motivation, my ritual is one hour out from the gym, I usually watch a vid of Yates, Coleman, Arnold, etc training a bodypart that I am about to train, and drink a Bang as I'm watching. An hour later I am mentally jacked and ready to kill some iron. :wallbash: